Vascular development
During mammalian embryogenesis, formation of the vascular system is an early and essential process. In the embryo, vascular development initiates with the pluripotent hemangioblast derived from the paraxial and lateral plate mesoderm. The hemangioblast has the potential to differentiate into either a hematopoietic progenitor or an endothelial cell progenitor, known as the angioblast.
Vascular development begins with a process known as vasculogenesis whereby angioblasts differentiate into endothelial cells and migrate together to form the primitive vascular plexus. This initial vascular network consists of vessels that are homogenous in size and made up wholly of endothelial cells. The vascular plexus is then remodeled via angiogenesis.
Angiogenesis involves the sprouting of new vessels, the migration of these vessels into avascular regions, and the recruitment of accessory cells, pericytes and smooth muscle cells (Gale and Yancopoulos, 1999). The smooth muscle cells that differentiate and form the contractile vessel walls originate from multiple progenitors including neural crest cells, mesenchymal cells and even endothelial cells (Owens, 1995). In adults, angiogenesis is involved in follicular development, wound healing, and pathological processes such as tumor angiogenesis and heart disease.
The Notch Family and Notch Ligands
Studies of Drosophila, C. Elegans, zebrafish and mammals have demonstrated that the Notch pathway is an evolutionarily conserved signaling mechanism that functions to modulate numerous cell-fate decisions. Notch signaling is required for the proper patterning of cells originating from all three germ layers. Depending on the cellular context, Notch signaling may both inhibit and induce differentiation, induce proliferation, and promote cell survival (Artavanis-Tsakonas et al., 1995; Lewis, 1998; Weinmaster, 1997). In Drosophila, a single Notch protein is activated by two ligands, Serrate and Delta. In mammals these families have been expanded to four Notch genes (Notch1, Notch2, Notch3 and Notch4) and five ligands, 2 Serrate-like (Jagged1-2) and 3 Delta (Dl1, 3, 4) (Bettenhausen et al., 1995; Dunwoodie et al., 1997; Gallahan and Callahan, 1997; Lardelli et al., 1994; Lindsell et al., 1995; Shawber et al., 1996a; Shutter et al., 2000a; Uyttendaele et al., 1996; Weinmaster et al., 1992; Weinmaster et al., 1991). During embryogenesis, Notch receptors and ligands are expressed in dynamic spatial and temporal patterns. However, it is not known if all ligands activate all receptors.
Notch Signaling and Function
Notch signaling influences many different types of cell-fate decisions by providing inhibitory, inductive or proliferative signals depending on the environmental context (reviewed in Artavanis-Tsakonas et al., 1995; Greenwald, 1998; Robey, 1997; Vervoort et al., 1997). This pleiotropic function suggests that Notch modulates multiple signaling pathways in a spatio-temporal manner.
Consistent with Notch regulating cell-fate decisions, both the receptors and ligands are cell surface proteins with single transmembrane domains (FIG. 1). The regulatory extracellular domain of Notch proteins consists largely of tandemly arranged EGF-like repeats that are required for ligand binding (Artavanis-Tsakonas et al., 1995; Weinmaster, 1998). C-terminal to the EGF-like repeats are an additional three cysteine-rich repeats, designated the LIN12/Notch repeats (LNR) (Greenwald, 1994). Downstream of the LNR lies the proteolytic cleavage sequence (RXRR) that is recognized by a furin-like convertase. For Notch1, cleavage at this site yields a 180 kilodalton extracellular peptide and a 120 kilodalton intracellular peptide that are held together to generate a heterodimeric receptor at the cell surface (Blaumueller et al., 1997; Kopan et al., 1996; Logeat et al., 1998).
The intracellular domain of Notch (NotchICD, FIG. 1) rescues loss-of-function Notch phenotypes indicating that this form of Notch signals constitutively (Fortini and Artavanis-Tsakonas, 1993; Lyman and Young, 1993; Rebay et al., 1993; Struhl et al., 1993).
The cytoplasmic domain of Notch contains three identifiable domains: the RAM domain, the ankyrin repeat domain and the C-terminal PEST domain (FIG. 1). Upon ligand-activation Notch undergoes two additional proteolytic cleavages which results in the release of the cytoplasmic domain (Weinmaster, 1998). This Notch peptide translocates to the nucleus and interacts with transcriptional repressors known as CSL (CBF, Su (H), Lag-2) and converts it to transcriptional activator. The CSL/Notch interaction is dependent on the presence of the RAM domain of Notch; while, transcriptional activity also requires the presence of the ankyrin repeats (Hsieh et al., 1996; Hsieh et al., 1997; Roehl et al., 1996; Tamura et al., 1995; Wettstein et al., 1997). Both in vivo and in vitro studies indicate that the HES and Hey genes are the direct targets of Notch/CSL-dependent signaling (Bailey and Posakony, 1995; Eastman et al., 1997; Henderson et al., 2001; Jarriault et al., 1995; Nakagawa et al., 2000; Wettstein et al., 1997). The HES and Hey genes are bHLH transcriptional repressor that bind DNA at N-boxes (Nakagawa et al., 2000; Sasai et al., 1992; Tietze et al., 1992). Notch has also been proposed to signal by a CSL-independent pathway. In fact, expression of just the ankyrin repeat domain is necessary and sufficient for some forms of Notch signaling (Lieber et al., 1993; Matsuno et al., 1997; Shawber et al., 1996b).
Finally, the PEST domain has been implicated in protein turnover by a SEL-10/ubiquitin-dependent pathway (Greenwald, 1994; Oberg et al., 2001; Rogers et al., 1986; Wu et al., 1998; Wu et al., 2001). Similar to the receptors, the extracellular domain of the Notch ligands also consist mostly of tandemly arranged EGF-like repeats (FIG. 1). Upstream of these repeats is a divergent EGF-like repeat known as the DSL (Delta, Serrate, Lag-2) that is required for ligand binding and activation of the receptors (Artavanis-Tsakonas et al., 1995).
Notch Signaling and Vascular Development
Although many of the genes that function to induce vasculogenesis and angiogenesis have been identified, little is known about how cell-fate decisions are specified during vascular development. A number of observations suggest that the Notch signaling pathway may play a role in cell fate determination and patterning of the vascular system.
Notch1, Notch4, Jagged1 and Dll4 are all expressed in the developing vasculature, while Notch3 is expressed in the accessory smooth muscle cells (Krebs et al., 2000; Shutter et al., 2000b; Uyttendaele et al., 1996; Villa et al., 2001; Xue et al., 1999). Mice lacking Jagged1 are embryonic lethal and have severe vascular defects (Xue et al., 1999). Mice nullizygous for Notch1 are embryonic lethal and die of severe neuronal defects, but also have defects in angiogenesis (Krebs et al., 2000; Swiatek et al., 1994). Mice lacking Notch4 are born and appear to be normal, but embryos that have lost both Notch1 and Notch4 die at E9.5 of severe hemorrhaging and vascular patterning defects indicating Notch1 and Notch4 may be functionally redundant during vascular development (Krebs et al., 2000). Exogenous expression of an activated form of Notch4 in endothelium also resulted in vascular defects similar to those seen for the double Notch1/Notch4 nullizygous mice, suggesting that appropriate levels of Notch signaling is critical for proper development of the embryonic vasculature (Uyttendaele et al., 2001).
Taken together, the data from mice mutant for Notch/Notch signaling components uncover several processes dependent on Notch including vascular remodeling, arterial venous specification, vascular smooth muscle cell recruitment and heart/heart outflow vessel development.
Recent experiments have implicated Notch signaling in arterial/venous endothelial cell specification. In situ analysis of E13.5 embryos found that Notch1, Notch3, Notch4, Dl4, Jagged1 and Jagged2 expression was restricted to the arteries and absent in the veins (Villa et al., 2001). Consistent with expression data, disruption of Notch signaling in Zebrafish was associated with loss of the arterial marker ephrinB2; while, ectopic expression of an activated form of Notch lead to a loss in the venous cell marker EphB4 within the dorsal aorta (Lawson et al., 2001). These data suggest that Notch signaling may help to specify arterial and venous cell fates during angiogenesis.
Taken together, the data from mice mutant for Notch/Notch signaling components uncover several processes dependent on Notch including vascular remodeling, arterial venous specification, vascular smooth muscle cell recruitment and heart/heart outflow vessel development.
Notch signaling has also been suggested to function in the adult vascular system. In humans, missense mutations in the extracellular domain of Notch3 correlate with the development of the degenerative vascular disease, CADASIL (Caronti et al., 1998; Desmond et al., 1998; Joutel et al., 2000; Joutel et al., 1996). In a wound healing model, an increase in Jagged1 expression was observed at the regenerating endothelial wound edge, suggesting Notch signaling may function during processes of adult angiogenesis (Lindner et al., 2001). Taken together these data support Notch signaling functions at a number of critical steps during vascular development: vasculogenesis, vascular patterning/angiogenesis, and arterial/venous specification. However, the molecular mechanism(s) by which the Notch signaling pathways influence these different steps has yet to be elucidated.
Significance
Shimizu et al. (J. Biol. Chem. 274(46): 32961-32969 (1999)) describe the use of Notch1ECD/Fc, Notch2ECD/Fc and Notch3ECD/Fc in binding studies. However, Shimizu et al. do not mention the use of such proteins for inhibiting angiogenesis.
U.S. Pat. No. 6,379,925 issued Apr. 30, 2002 to Kitajewsky et al. describes murine Notch4. However, it does not describe Notch-based fusion proteins as set forth in the subject application.
This invention differs from the prior art because it is the first study using Notch-based fusion proteins comprising the extracellular domain of Notch operably affixed to a half-life-increasing moiety to inhibit angiogenesis. This invention therefore provides an advantage over the prior art in that it provides evidence that such Notch-based fusion proteins are capable of inhibiting angiogenesis.
Notch proteins play key roles in developmental decisions involving the vasculature, the hematopoietic system, and the nervous system. As such, an understanding of their function is key to understanding how cell-fate decisions and commitment are controlled during development and in adult tissues. To date, several reports on Notch or Notch ligand gene disruptions have described vascular phenotypes providing emphasis that this pathway is a fundamental part of the machinery that guides vascular development. Aberrant Notch activity has been linked to human pathologies; including both cancer and vascular disorders (CADASIL). The analysis of Notch in tumor angiogenesis has only recently begun; however, our discovery of potential downstream targets of Notch suggests a roles in pathological processes associated with angiogenesis. For instance, VEGFR-3 has been linked to both tumor angiogenesis and tumor lymphangiogenesis. The expression or function of several other potential Notch targets has also been linked to tumor angiogenesis; including ephrinB2, Id3, Angiopoietin 1, and PDGF-B. Insights on the role of these targets in Notch gene function will clearly facilitate future analysis of Notch in human pathologies.